1. Technical Field
Several aspects of the present invention relate to a semiconductor film, a semiconductor element, a semiconductor device, and methods of manufacturing them.
2. Related Art
Polysilicon thin film transistor (poly-Si TFT) is used extensively as means for manufacturing high-performance transistor elements on insulating substrates made of glass, quartz, or the like. But, structural defects in the polysilicon thin film strongly affect the performance of the poly-Si TFT. In the polysilicon thin film, there are various kinds of structural defects such as implantation, twin boundary, stacking fault, or grain boundary. Poly-Si TFTs are generally inferior in performance to single crystal silicon elements because these defects prevent electrons/holes from moving in electrical conduction. In order for solving such a problem, a method of enlarging the grain size in the polysilicon thin film on the insulating substrate or a method of partially forming a quasi-single crystal silicon thin film has been reported. Such methods will be listed below.
A sequential lateral solidification (SLS) method is a method for obtaining a polysilicon thin film by irradiating with excimer laser wherein the crystal grains are elongated in the laser scanning direction by setting the pitch in the scanning direction to be extremely small (see, for example, “Characterization of poly-Si TFTs in Directionally Solidified SLS Si” by S. D. Brotherton, et al., Asia Display/IDW '01 Proceedings, pp. 387-390).
A CW-laser lateral crystallization (CLC) method is a method of scanning a substrate with continuous oscillation laser irradiating thereon thereby elongating silicon crystal grains in the laser scanning direction (see, for example, “Ultra-high Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization,” A. Hara, et al., AM-LCD '01, Digest of Technical Papers, pp. 227-230).
A selectively enlarging laser X'tallization (SELAX) method is a method of performing crystallization with excimer laser and then elongating the existing crystal grains in the laser scanning direction using the continuous oscillation laser (see, for example, “System on Glass Display with LTPS-TFTs Formed using SELAX (Selectively Enlarging Laser X'tallization) Technology,” M. Hatano, et al., Proceedings of IDW/AD '05, pp. 953-956).
A phase-mask modulated excimer laser annealing (PMELA) method is a method of forming an excimer laser beam having intensity distribution using a phase mask to perform crystallization of the silicon thin film and making the crystal grains grow in the longitudinal direction using thermal gradient caused between the high intensity area and the low intensity area (see, for example, “Advanced Phase-Modulators for Next-Generation Low-Temperature Si Film Crystallization Method,” Y. Taniguchi, et al., Proceedings of IDW/AD '05, pp. 981-982).
A μ-Czochralski method is a method of providing fine holes on a substrate, depositing amorphous silicon thin film so as to cover the fine holes, and irradiating this structure with the excimer laser to promote meltdown/crystallization from the bottoms of the fine holes, thereby selectively allowing only crystals having the fastest growth rate to grow, thus making it possible to obtain a quasi-single crystal silicon thin film (see, for example, “Effects of crystallographic orientation of single-crystalline seed on μ-Czochralski process in excimer-laser crystallization,” M. He, et al., proceedings of IDW/AD '05, pp. 1213-1214, and “Single-Crystalline Si Thin-Film Transistors Fabricated with μ-Czochralski (Grain-Filter) process,” R. Ishihara, et al., AM-LCD '02 Digest of Technical Papers, pp. 53-56).
By using these methods, a polysilicon thin film including crystal grains with a grain size of greater than several micrometers can be obtained in each of the cases. By manufacturing thin film transistor elements to such a silicon thin film while paying attention to not including a crystal grain, a thin film transistor element having a carrier mobility as high as 300 through 500 Cm2/Vs or more can be obtained on the insulating substrate in each case.
However, although crystal grains as large as several micrometers can be formed by the method of the related art described above, the crystal orientations of the obtained crystal grains are not controlled, and accordingly, are left in random conditions. Since the carrier mobility is varied depending on the crystal orientation of silicon, the performance is widely varied among the thin film transistors having crystal orientations not unified with each other. In order for achieving further improvement of the electrical characteristics of thin film transistor elements, establishment of a manufacturing method capable of forming a high-quality semiconductor thin film with crystal grains having controlled crystal orientations is desired.